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Abstract

We have developed a multimodal multiphoton laser-scanning microscope for cell imaging featuring simultaneous acquisition of differential Coherent Antistokes Raman Scattering (D-CARS), two-photon fluorescence (TPF) and second harmonic generation (SHG) using a single 5 fs Ti:Sa broadband (660–970 nm) laser. The spectral and temporal pulse requirements of these modalities were optimized independently by splitting the laser spectrum into three parts: TPF/SHG excitation (> 900 nm), CARS Pump excitation (< 730 nm), and CARS Stokes excitation (730–900 nm). In particular, by applying an equal linear chirp to pump and Stokes pulses using glass dispersion we achieved a CARS spectral resolution of 10 cm−1, and acquired CARS images over the 1200–3800 cm−1 vibrational range selected by the time delay between pump and Stokes. A prism pulse compressor in the TPF/SHG excitation was used to achieve Fourier limited 30 fs pulses at the sample for optimum TPF and SHG. D-CARS was implemented with few passive optical elements and enabled simultaneous excitation and detection of two vibrational frequencies with a separation adjustable from 20 cm−1 to 150 cm−1 for selective chemical contrast and background suppression. The excitation/detection set-up using beam-scanning was built around a commercial inverted microscope stand providing conventional bright-field, differential interference contrast and epi-fluorescence for user-friendly characterization of biological samples. Examples of CARS hyperspectral images and simultaneous acquisition of D-CARS, TPF and SHG images in both forward and epi-direction are shown on HeLa cells, stem-cell derived human adipocytes and mouse tissues.

Simultaneously acquired images for TPE (top row), pump and Stokes excitation at 2850 cm−1 (middle row), and CARS with TPE (bottom row) of the mouse tail section in Fig. 4. Linear grey scale over the range shown at the top of each column in photoelectrons/sec at the PMT cathode. Scale bar 20 μm.